Working color space

The camera gamut setting only affects the in-camera JPEG, not the Raw file.

true, but then agian, the raw isn't a bitmap file, it has no need for a colorspace.

The main reason to use ProPhoto RGB as a working space is that it is 16-bit.

untrue, in the first place because bit depth is a file-property, not a propertie of a colourspace... any RGB space can handle 8bit or 16bit images. You can convert the raw file from your camera into either one using a raw developer. If you let your camera make the JPG, it will be 8 bit only, because JPG as a file cannot handle 16 bits of information.

Your Raw file is either 12 or 14 bits, and the output file (aRGB or sRGB) will be 8 bits. You want a higher working space resolution so you don't get quantization errors (= noise, color shifts and banding) as you process the image.

'resolution' isn't the word used for that, it's bitdepth. It describes the number of values a piece of information can have. in 8bit images, every pixel has color descriptions for 3 channels (R, G and B, hence the name), that each can have a value between 0 and 255 (2^8 bits). In a 16 bit image, each color description can have a value between 0 and 16,7 milion. (2^16 bits) When you go and edit such a picture it will be less prone to posterisation or banding.